Heat treatment and stress relief for solid-state welded nickel alloys

ABSTRACT

A joining method includes performing a first heat treatment step on a first superalloy workpiece and a second superalloy workpiece wherein at least one of the first and second superalloy workpieces include a gamma matrix phase and a gamma-prime precipitate phase. The first and second superalloy workpieces are joined using a solid state joining process, subjected to a post-weld stress relief operation and a final aging heat treatment.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with Government support underFA8650-09-D-2923-0021 awarded by United States Air Force. The Governmenthas certain rights in this invention.

BACKGROUND

The disclosure relates generally to joining and heat treating alloys,and more specifically to heat treatment and stress relief in nickelalloys.

Components using nickel-based powder metallurgy superalloys aregenerally fabricated using solid-state welding processes, such asinertial friction welding, due to weldability issues associated withfusion welding. These alloys are often welded in the fully heat treatedcondition (i.e. solution, stabilization, and age), followed by a postweld stress relief (PWSR) after joining. Temperatures and times during aPWSR cycle, however, can result in over-aging of the gamma-primestrengthening precipitates in the base material, thereby resulting in areduction in base metal strength in some cases. This reduction instrength can influence the design or limit the operational parametersduring life of the part.

SUMMARY

A joining method includes performing a first heat treatment step on afirst superalloy workpiece and a second superalloy workpiece wherein atleast one of the first and second superalloy workpieces include a gammamatrix phase and a gamma-prime precipitate phase. The first and secondsuperalloy workpieces are joined using a solid state joining process,subjected to a post-weld stress relief operation, and a final aging heattreatment.

An embodiment of a welded structure includes a weld region joining afirst superalloy workpiece and a second superalloy workpiece. The firstand second superalloy workpieces include respective first and secondbase material regions including a solution-treated gamma matrix phaseand a gamma-prime precipitate phase. The weld region includes aheat-affected zone of the first and second superalloy workpiecesadjacent to a portion of the first and second base material regions. Theweld region, including the heat affected zone, is not stress-relievedsuch that the portions of the first and second base material regions,adjacent to the heat affected zone, are not aged.

An embodiment of a welded structure includes a weld region joining afirst superalloy workpiece and a second superalloy workpiece. The firstand second superalloy workpieces include respective first and secondbase material regions including a solution-treated gamma matrix phaseand a gamma-prime precipitate phase. The weld region includes aheat-affected zone of the first and second superalloy workpiecesadjacent to a portion of the first and second base material regions. Theweld region, including the heat affected zone, includes a post-weldstress relief region such that the portions of the first and second basematerial regions, adjacent to the heat affected zone, are not overaged.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a prior art heat treatment flow chart for nickel-basedsuperalloys.

FIG. 2 is a prior art flow chart for heat treating welded nickel-basedsuperalloys.

FIG. 3 is a first flow chart for heat treating and joining nickel-basedsuperalloys according to the disclosure.

FIG. 4 is a second flow chart for heat treating and joining nickel-basedsuperalloys according to the disclosure.

DETAILED DESCRIPTION

FIG. 1 outlines an accepted thermal processing and heat treatmentschedule 10 for advanced nickel-based superalloys, such as those used ingas turbine engine applications. In step 12, typically occurring soonafter forging, the alloys are first subjected to a super-solvus solutionheat treatment in excess of about 1900° F. (1039° C.), or otherwiseabove the gamma-prime solvus temperature of a particular alloy for atime sufficient to dissolve strengthening phases such as gamma-primewhich were precipitated irregularly during initial solidification. Thistemperature is alloy dependent, although around 2000° F. and above,super-solvus heat treatment can result in grain coarsening due toabsence of gamma-prime precipitates to pin grain boundaries. Holdingtimes of a few hours at temperature at or immediately above the solutiontemperature may be sufficient to achieve desired phase solution. Aircooling or faster cooling is then performed to re-precipitate a refinedmicrostructure of strengthening gamma-prime phase during cool down.

In the next step, the alloy is typically subjected to a fullstabilization (step 14) heat treatment at an elevated temperature. Afull stabilization, immediately after solution treatment, may form orgrow carbides and borides on grain boundaries to increase grain boundarystrengthening, in addition to enabling a determined amount ofgamma-prime growth. A stabilization cycle as part of step 14 is alsonecessary to provide residual stress relief accumulated during forgingand solution heat treatment. Some stabilization heat treatmenttemperatures are in the vicinity of 1500° F. (816° C.). Around thistemperature, a cycle time on the order of 4 hours is necessary for fullstabilization to occur. As taught by commonly owned U.S. Pat. No.7,708,846 to Malley, incorporated herein in its entirety by reference,shorter duration, higher temperature full stabilization cycles had beenfound to improve creep and stress rupture properties of bothcast/wrought and powder metal forms of nickel based super alloys. Thecombination of time and temperature also further stabilizes themicrostructure by spheroidizing and reducing the size of carbides andborides formed in the microstructure during step 14. Exemplarytemperatures for the accelerated stabilization treatment are about 1800°F. (982° C.) and exemplary times are about 1 hour.

Finally, the precipitation hardened and stabilized alloy is thentypically subjected to aging (step 16), which results in controlledaging of gamma-prime strengthening phase in the gamma phase matrix andprovides more relief of accumulated residual stresses. This temperaturewill be alloy dependent and application specific. Differentstabilization and aging temperatures can be selected for these alloys inorder to optimize specific material properties (e.g. creep). Aging atabout 1350° F. (732° C.) for a time on the order of eight hours istypical but parameters can be varied in time and/or temperature for agiven composition.

Prior art joining and heat treatment process 20 is shown in FIG. 2. Whenjoining one or more nickel-based superalloy components or workpieces,all of the conventional heat treatment steps are performed prior tojoining. Joining such as through friction welding or other forms ofsolid-state welding are known to subsequently require a post weld stressrelief (PWSR) cycle, performed at least around the weld region.

Thus in a typical joining and heat treatment process for nickel-basedsuperalloy components, the superalloy is first solution heat treated(step 22), fully stabilized (step 24), and aged (step 26) as describedabove. Components are then joined such as by a solid state or otherwelding process (step 28). The PWSR treatment may include a hightemperature stabilization treatment for between one-half the normalstabilization time, such as 2 hours to 4 hours, up to a fullstabilization treatment (step 30), also as described with respect toFIG. 1. These ranges will be alloy dependent, as it depends on thestress relaxation behavior of the weld material during PWSR. Thepost-weld stabilization 30 can be followed by a normal aging treatment(step 32). The process outlined in FIG. 2 results in an over-aging ofthe base material around the weld, adjacent to and including the heataffected zone (HAZ). While the HAZ contains gamma-prime phase, theprecipitate structure is much finer than in the base metal. Therefore,the PWSR will likely result in property improvement within the weld dueto precipitation and coarsening of gamma-prime precipitates, whereas adegradation in base metal properties occurs due to over-aging of thegamma-prime precipitates formed during the initial full heat treatmentprocess. This over-aging is due to added time at elevated temperature,since the base metal near the weld has effectively been exposed tomultiple stabilization and aging processes reducing the mechanicalproperties and microstructures as compared to the unwelded components.In certain prior art processes, depending on alloy composition, thealloy can undergo constitutional liquation near its grain boundariesduring the welding process as described below.

An appropriate heat treatment and stress relief process 40 for weldednickel superalloys is shown in FIG. 3. Candidate alloys for the joiningprocesses of the present disclosure can include coarse grain powderalloys, such as but not limited to IN100, ME3, NF3, LSHR, PRM48, Rene88, Rene 95, and/or RR1000.

Process 40 starts with both superalloy components or workpieces beingsubjected to a solution heat treatment (step 42). In one example,gamma-prime strengthening phases from the initial solidification processare dissolved by heating the workpieces above a solvus temperature inexcess of about 1900° F. (1038° C.) for holding times of about a fewhours as described earlier with respect to FIG. 1. In the next step(44), both components are subjected to a partial high temperaturestabilization heat treatment. A partial stabilization, for up to onehalf the time required for a full stabilization treatment, can beperformed to provide stress relief prior to welding to preventdistortion during the weld process. Partial stabilization permits somerelief of accumulated residual stress from forging and solution heattreatment to prevent distortion during inertia weld. It also allows theprecipitation and growth of gamma-prime precipitates in the gammamatrix, providing some coherency strain between the matrix and theprecipitates during the welding stage.

In one example of a partial standard stabilization, thecomponents/workpieces are heated to a vicinity of 1500° F. (816° C.) butfor no more than 2 hours. In another example, accelerated stabilizationcan be performed at elevated temperature on the order of 1800° F. (982°C.) but for no more than ½ hour. This also minimizes over aging the basemetal structure following final thermal processing steps describedbelow. In certain embodiments (e.g., FIG. 4), stabilization treatment isnot performed at all prior to joining.

In the next step (46), the components are joined by solid state welding.Since joining fully heat treated advanced nickel based superalloycomponents by fusion welding is not suitable because fusion weldingresults not only in solidification cracking of these alloys, but strainage cracking also frequently occurs during post-fusion weld stressrelief. Even with solid state welding procedures, strain age crackingcan be an issue for these types of alloys. A non-limiting example of asolid state welding process is friction welding, which minimizes theheat affected zone in and around the join area. Other acceptable solidstate welding procedures include linear friction welding, friction plugwelding, and rotary friction welding.

Following welding, step 48 includes subjecting the structure to astabilization heat treatment. This can include at least the remainder ofthe stabilization heat treatment which would have otherwise beenperformed at step 44. It can alternatively include a full stabilizationprocess (either standard, accelerated, or otherwise). As noted above,stabilization heat treatments are necessary to develop a microstructurewith superior elevated temperature stability in service and providestress relief.

Part of the stabilization and aging processes results in modification ofthe gamma-prime precipitate structure, as well as borides, carbides,etc., A boride or carbide particle, if intersected by a grain boundaryat high temperatures, (e.g., as seen during welding) will devolve intoconstituent elements including boron and carbon. In sufficientquantities, these constituent elements can operate as melting pointdepressants and may therefore result in grain boundary liquation in theHAZ during welding. But by limiting the time of an initial hightemperature stabilization treatment, or omitting it altogether (see FIG.4), one can minimize the presence of grain boundary borides and carbidesduring the welding step, and in turn minimize the occurrence ofliquation during welding.

Following the second stabilization heat treatment, the welded structureis given an aging treatment to modify the structure of gamma-primestrengthening phase in the gamma matrix to form a fully heat treatedsuperalloy structure (step 50). As mentioned earlier, an acceptableaging heat treatment may be 1350° F. (732° C.) for 8 hours followed byan air cool.

In other embodiments designed to minimize liquation in solid state weldsin superalloys of the present invention, the high temperaturestabilization heat treatment may be postponed until after the structureis welded. Process 60 shown in FIG. 4 adapts this procedure. As before,components of a superalloy structure designated for joining by solidstate welding are subjected to a solution heat treatment to dissolveexisting gamma-prime strengthening phase by heating to about 1900° F.(1038° C.) for a time sufficient to dissolve gamma-prime (step 62). Inaddition to dissolving gamma-prime, minor phases containing highconcentrations of melting point suppressing species such as borides arealso dissolved. Holding times of a few hours may be sufficient asmentioned earlier. Quenching can also prevent the formation of boridephases, preventing the formation of phases that are susceptible toliquation during welding. In the embodiment, the fully solution heattreated components are joined by solid state welding (step 64). Apreferred solid state welding process is friction welding.

After welding, the joined structure is subjected to a full stabilizationtreatment (step 66). This may be a standard stabilization or anaccelerated high temperature stabilization treatment of about 1800° F.(982° C.) for about one hour or as otherwise described above, therebymodifying grain boundary carbides, borides, or other particles in themicrostructure and to further stabilize the microstructure. Finally, thewelded structure is given an aging heat treatment (step 68) such as isdescribed in earlier processes (e.g., at about 1350° F./732° C.) forabout 8 hours) to grow very fine gamma-prime throughout themicrostructure, as well as provide additional stress relief.

The result of the processes described broadly in FIGS. 3 and 4 is awelded structure with at least first and second superalloy workpieces,one or both having respective first and second base material regionsincluding a solution-treated gamma matrix phase and a gamma-primeprecipitate phase. The workpieces are joined at a weld region to formthe welded structure. The weld region includes a heat-affected zone ofthe first and second superalloy workpieces adjacent to a portion of thefirst and second base material regions.

Prior to welding, the weld region, including the heat affected zone, isnot stress-relieved such that the base material regions adjacent to theheat affected zone are not aged, prior to a post-weld stress relief. Asnoted with respect to the prior art process of FIG. 2, conventionalsolid state welded superalloy structures undergo a full heat treatmentincluding full stabilization and aging processes immediately prior tothe joining process. Full stabilization results in borides, carbides, orother grain boundary strengtheners, derived from one or more constituentelements operative to reduce the melting temperature of the gamma matrixphase being formed in the weld region, including in a portion of thebase material regions around the heat affected zone. And the fully agedmaterial, following a post-weld stress relief that includes a secondaging step, can result in overaging of the base material around the heataffected zone, weakening it as compared to the rest of the basematerial. The overaged base material is much more susceptible to strainage cracking as compared to the rest of the base material.

Here, following the processes in FIGS. 3 and 4, the base materialregions adjacent to the heat affected zone are partially but not fullystabilized before welding, avoiding accumulation of one or moreconstituent elements (e.g., boron or carbon) at or proximate a pluralityof grain boundaries adjacent to the heat affected zone. As a result,after welding, the weld region joining the first and second superalloyworkpieces, including a heat-affected zone of the first and secondsuperalloy workpieces adjacent to a portion of the first and second basematerial regions, a post-weld stress relieved region such that the basematerial region adjacent to the heat affected zone that is not overaged,and has comparable or equivalent mechanical properties to the remainderof each corresponding base material region. This facilitates weldedsuperalloy components such as, but not limited to rotor components,bladed disks, and shaft components for turbine engines.

Discussion of Possible Embodiments

The following are non-exclusive descriptions of possible embodiments ofthe present invention.

A joining method includes performing a first heat treatment step on afirst superalloy workpiece and a second superalloy workpiece wherein atleast one of the first and second superalloy workpieces include a gammamatrix phase and a gamma-prime precipitate phase. The first and secondsuperalloy workpieces are joined using a solid state joining process,subjected to a post-weld stress relief operation, and a final aging heattreatment.

The method of the preceding paragraph can optionally include,additionally and/or alternatively, any one or more of the followingfeatures, configurations and/or additional components:

A joining method according to an exemplary embodiment of thisdisclosure, among other possible things includes performing a first heattreatment step on a first superalloy workpiece and a second superalloyworkpiece, at least one of the first and second superalloy workpiecescomprising a gamma matrix phase and a gamma-prime precipitate phase;metallurgically joining the first and second superalloy workpieces usingat least one solid-state joining process; and performing a post-weldstress relief operation on the joined first and second superalloyworkpieces; wherein the first heat treatment step excludes an aging heattreatment.

A further embodiment of the foregoing method, wherein the at least oneof the first and second superalloy workpieces further comprises a firstconstituent element operative to reduce a melting temperature of thegamma phase.

A further embodiment of any of the foregoing methods, wherein the firstconstituent element comprises boron or carbon.

A further embodiment of any of the foregoing methods, wherein the firstconstituent element is entirely dispersed in the gamma phase after thefirst heat treatment step.

A further embodiment of any of the foregoing methods, wherein the firstheat treatment step consists of a solution heat treatment.

A further embodiment of any of the foregoing methods, wherein themetallurgical joining step is performed after the first heat treatmentstep without an intervening second heat treatment step.

A further embodiment of any of the foregoing methods, further comprisinga second heat treatment step after the first heat treatment step andprior to the metallurgical joining step, the second heat treatment stepexcluding an aging heat treatment.

A further embodiment of any of the foregoing methods, wherein the secondheat treatment step consists of a partial stabilization step terminatingprior to substantial accumulation of the first constituent element at orproximate a plurality of grain boundaries in an area of the first andsecond workpieces to be joined during the metallurgical joining step.

A further embodiment of any of the foregoing methods, wherein thepost-weld stress relief operation comprises a third heat treatment stepperformed on a heat affected zone after the metallurgical joining step,the third heat treatment step consisting of a partial stabilization stepterminating after stress relief of the heat affected zone, and prior tomodification of a microstructure of a base metal adjacent to the heataffected zone.

A further embodiment of any of the foregoing methods, wherein thepost-weld stress relief operation further comprises a fourth heattreatment step performed on the heat affected zone after the third heattreatment step, the fourth heat treatment step consisting of an agingheat treatment.

A further embodiment of any of the foregoing methods, wherein the firstand second superalloy workpieces comprise at least one of: rotorcomponents, bladed disks, and shaft components.

An embodiment of a welded structure includes a weld region joining afirst superalloy workpiece and a second superalloy workpiece. The firstand second superalloy workpieces include respective first and secondbase material regions including a solution-treated gamma matrix phaseand a gamma-prime precipitate phase. The weld region includes aheat-affected zone of the first and second superalloy workpiecesadjacent to a portion of the first and second base material regions. Theweld region, including the heat affected zone, is not stress-relievedsuch that the portions of the first and second base material regions,adjacent to the heat affected zone, are not aged.

The structure of the preceding paragraph can optionally include,additionally and/or alternatively, any one or more of the followingfeatures, configurations and/or additional components:

A welded structure according to an exemplary embodiment of thisdisclosure, among other possible things includes a first superalloyworkpiece; a second superalloy workpiece comprising respective first andsecond base material regions including a solution-treated gamma matrixphase and a gamma-prime precipitate phase; and a weld region joining thefirst and second superalloy workpieces to form the welded structure, theweld region including a heat-affected zone of the first and secondsuperalloy workpieces adjacent to a portion of the first and second basematerial regions; wherein the weld region, including the heat affectedzone, is not stress-relieved such that the portions of the first andsecond base material regions adjacent to the heat affected zone are notaged.

A further embodiment of the foregoing structure, wherein a firstconstituent element is operative to reduce the melting temperature ofthe gamma matrix phase.

A further embodiment of any of the foregoing structures, wherein thefirst constituent comprises boron or carbon.

A further embodiment of any of the foregoing structures, wherein thebase material regions adjacent to the heat affected zone are partiallybut not fully stabilized avoiding accumulation of the first constituentelement at or proximate a plurality of grain boundaries adjacent to theheat affected zone.

An embodiment of a welded structure includes a weld region joining afirst superalloy workpiece and a second superalloy workpiece. The firstand second superalloy workpieces include respective first and secondbase material regions including a solution-treated gamma matrix phaseand a gamma-prime precipitate phase. The weld region includes aheat-affected zone of the first and second superalloy workpiecesadjacent to a portion of the first and second base material regions. Theweld region, including the heat affected zone, includes a post-weldstress relief region such that the portions of the first and second basematerial regions, adjacent to the heat affected zone, are not overaged.

The structure of the preceding paragraph can optionally include,additionally and/or alternatively, any one or more of the followingfeatures, configurations and/or additional components:

A welded structure according to an exemplary embodiment of thisdisclosure, among other possible things includes a first superalloyworkpiece; a second superalloy workpiece comprising respective first andsecond base material regions including a solution-treated gamma matrixphase and a gamma-prime precipitate phase; and a weld region joining thefirst and second superalloy workpieces to form the welded structure, theweld region including a heat-affected zone of the first and secondsuperalloy workpieces adjacent to a portion of the first and second basematerial regions; wherein the weld region includes a post-weld stressrelieved region such that the portions of the first and second basematerial regions adjacent to the heat affected zone are not overaged.

A further embodiment of the foregoing structure, wherein the portions ofthe base material regions adjacent to the heat affected zone are atleast partially stabilized and fully aged but not overaged.

A further embodiment of any of the foregoing structures, wherein thesuperalloy workpieces comprise at least one of: rotor components, bladeddisks, and shaft components.

While the invention has been described with reference to an exemplaryembodiment(s), it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment(s) disclosed, but that theinvention will include all embodiments falling within the scope of theappended claims.

The invention claimed is:
 1. A joining method comprising: performing afirst heat treatment step on a first superalloy workpiece and a secondsuperalloy workpiece, at least one of the first and second superalloyworkpieces comprising a gamma matrix phase and a gamma-prime precipitatephase, and further comprises a first constituent element operative toreduce a melting temperature of the gamma phase; metallurgically joiningthe first and second superalloy workpieces using at least onesolid-state joining process; a second heat treatment step after thefirst heat treatment step and prior to the metallurgical joining step;and performing a post-weld stress relief operation on the joined firstand second superalloy workpieces; wherein the first heat treatment stepexcludes an aging heat treatment and includes a solution heat treatment;wherein the first constituent element is entirely dispersed in the gammaphase after the first heat treatment step; and wherein the second heattreatment step excludes an aging heat treatment and includes a firstpartial stabilization step terminating prior to substantial accumulationof the first constituent element at or proximate a plurality of grainboundaries in an area of the first and second workpieces to be joinedduring the metallurgical joining step, substantial accumulation beingless than an amount sufficient to cause overaging of the respectiveareas to be joined.
 2. The joining method of claim 1, wherein the firstconstituent element comprises boron or carbon.
 3. The joining method ofclaim 1, wherein the post-weld stress relief operation comprises a thirdheat treatment step performed on a heat affected zone after themetallurgical joining step, the third heat treatment step consisting ofa second partial stabilization step terminating after stress relief ofthe heat affected zone, and prior to modification of a microstructure ofa base metal adjacent to the heat affected zone.
 4. The joining methodof claim 3, wherein the post-weld stress relief operation furthercomprises a fourth heat treatment step performed on the heat affectedzone after the third heat treatment step, the fourth heat treatment stepconsisting of an aging heat treatment.
 5. The joining method of claim 1,wherein the first and second superalloy workpieces comprise at least oneof: rotor components, bladed disks, and shaft components.
 6. The joiningmethod of claim 1, wherein the first partial stabilization stepterminates prior to modification of a microstructure of a base metaladjacent to the heat affected zone.